[WIP] Leave/Close

Discussion of concecpt in #47.
Discussion of code here.

 - [x] new table
 - [x] new entity + mapper
 - [x] new command
 - [x] add command to client
 - [x] insert initconv on join
 - [x] implement client
 - [x] Fetch initconvs from new table
 - [x] Adjust the Invite command so only invite commands are send to sessionIds not in the conv
 - [x] client: fetch previous msgs when startConving a new conv